1. Field of the Invention
The invention relates to an oil system for a brake disk having brake linings with generally radially oriented grooves for carrying oil from the inside to the outside of the disk, and which have at least one radially outer oil collection channel from which the oil returns through at least one return passage.
2. Description of the Related Art
Many wet brakes have brake linings with generally radially oriented grooves to carry oil from the radially inner portion of the liner to the radially outer portion of the liner. The brake linings also are often provided with through passages to allow the oil level on both sides of the disk to equalize.
Oil is provided in the brake housing, and the brake disk is usually partially submerged in the oil so that the radially inner openings of the grooves are submerged in the oil bath. The oil flows radially out along the grooves to the outside of the disk due to the centrifugal forces resulting from rotation of the brake disk. Some oil flows out of the grooves of the brake lining to coat the contact surfaces of the brake to lubricate and cool it, which in turn heats the oil.
The oil collects in a radially outer collection channel, and flows from there through return passages to the brake housing sump. Alternatively, the oil may pass through an oil reservoir or oil cooler before returning to the brake housing sump. Once in the brake housing, the warm oil mixes with the remaining oil, cools, and is again picked up by the rotating grooves, so that a cooling oil circuit is established.
The oil flowing out of the grooves of the brake disk loses its pressure in the oil collection channel, and flows unpressurized through the return passages. The return passages therefore normally have a relatively large cross-sectional area to ensure an unhampered flow of oil from the collection channel. For example, the return passages in the service brake of an agricultural tractor may include three equally spaced radial holes with diameters of about 6 mm.
It has been found that such an oil system does not always assure optimum lubrication and cooling for the brake, particularly when the brake disk is operating at high rotational speeds.
Above a certain critical rotational speed (which varies from brake to brake), the velocity of the oil flow in the grooves becomes so great that a greater quantity of oil is ejected than can be supplied. The oil flow at the ends of the grooves breaks down and air or an air/oil mixture is ingested, creating foam. In addition, air is entrained in the oil in the vicinity of the brake disk due to its high rotational speed. The air entrainment is reinforced if the brake disk is provided with the usual through passages to allow the oil the oil level to equalize on both sides of the brake disk.
The amount of air entrained in the oil or carried along as an air/oil foam can be so large that it can cause localized dry friction, high temperatures, combustion of the oil and destruction of the contact surfaces or brake lining during the braking process.